The present invention relates to the forming of multilayer ceramic laminated structures and, more particularly, to the forming of multilayer ceramic laminated structures having a ceramic coating thereon for camber modification during sintering.
Multilayer ceramic (MLC) structures are used in the production of electronic substrates and devices. The MLCs can have various layering configurations. For example, an MLC circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as a dielectric medium. For the purposes of interlayer interconnections, most of the ceramic layers have via holes, more typically called vias. Prior to lamination, the vias are typically filled with an electrically conductive paste, such as a metallic paste, and sintered to form vias which provide the electrical connection between the layers. In addition, the MLC substrates may have termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, to name a few.
Generally, conventional ceramic structures are formed from ceramic greensheets which are prepared from a slurry of ceramic particulate, thermoplastic polymer binders, plasticizers, and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible greensheets. After via formation, metal paste screening, stacking and lamination, the greensheets are fired or sintered at temperatures sufficient to burn-off or remove the unwanted polymeric binder resin and sinter the ceramic particulate together into a densified ceramic substrate.
Camber is the slight curving of the MLC substrate so that it is not flat. MLC substrate camber occurring during sintering is generated by the difference in densification behavior between the ceramic powder and the substrate metallurgy, or by differences in the green density of the ceramic body after the lamination process. MLC substrates with camber some times can be reworked by a liquid-phase sintering process or, in the case of glass-based ceramic systems, by a flattening process but at additional processing cost. Moreover, the flattening process can generate defects on the substrate surfaces which translate into additional processing cost from inspection and yield losses. Hence, camber control prior to substrate post-sinter processing is desired.
Various solutions have been proposed to reduce camber.
Dubetsky et al. U.S. Pat. No. 4,340,436, the disclosure of which is incorporated by reference herein, discloses a two-step sintering process wherein the sintering process for a ceramic substrate is interrupted, a light weight placed on the ceramic substrate and then the sintering process is continued. The result is a flat ceramic substrate.
Bastian et al. U.S. Pat. No. 5,874,162, the disclosure of which is incorporated by reference herein, discloses a conformable load tile which is placed over the ceramic substrates. During the sintering process, the conformable load tile deforms to maintain flatness of the ceramic substrates.
The aforementioned solutions to the camber problem are mechanical in nature. It would be desirable to have a solution to the camber problem that is not mechanical in nature.
Generally speaking, it is known to apply coatings to ceramic substrates for various purposes. For example, Kumagai et al. U.S. Pat. No. 5,034,260, the disclosure of which is incorporated by reference herein, discloses the addition of a coating to a ceramic substrate for the purpose of insulating crystal grain boundaries.
It is a purpose of the present invention to have a method for controlling the camber of a ceramic substrate which is not mechanical in nature.
It is another purpose of the present invention to have a method for manufacturing a ceramic substrate with a coating which controls the camber of the ceramic substrate.
It is yet another purpose of the present invention to have a ceramic substrate with a coating which controls the camber of the ceramic substrate.
These and other purposes of the invention will become more apparent after considering the following description of the invention in conjunction with the accompanying Figures.
A first aspect of the present invention relates to a method of forming a multilayer ceramic substrate with reduced camber, the method comprising the steps of:
forming a plurality of ceramic greensheets;
stacking and laminating the plurality of ceramic greensheets to form a stack with one of the plurality of ceramic greensheets being a first ceramic greensheet of the stack and one of the plurality of ceramic greensheets being a last ceramic greensheet of the stack;
forming a coating of ceramic material on at least one of the first and last ceramic greensheets of the stack;
sintering the stack of ceramic greensheets at a predetermined temperature to cause densification of the ceramic material in the ceramic greensheets;
wherein the coating of ceramic material on at least one of the first and last ceramic greensheets of the stack alleviates camber resulting from sintering.
A second aspect of the present invention relates to a method of forming a multilayer ceramic substrate with reduced camber, the method comprising the steps of:
forming a plurality of ceramic greensheets wherein the ceramic material of the ceramic greensheets primarily comprises alumina particles, glass ceramic particles or aluminum nitride particles;
applying a coating of ceramic material to at least one of the plurality of ceramic greensheets wherein the ceramic material of the coating is glass frit, grit, magnesium silicate, aluminum-magnesium silicate, aluminum-magnesium-silicon oxide, calcia-alumina-boria glass, or calcia-alumina-boria-yttria glass;
stacking and laminating the plurality of ceramic greensheets to form a stack;
sintering the stack of ceramic greensheets at a predetermined temperature to cause densification of the ceramic material in the ceramic greensheets;
wherein the coating of ceramic material on at least one of the plurality of ceramic greensheets alleviates camber resulting from sintering.
A third aspect of the present invention relates to a method of forming a multilayer ceramic substrate with reduced camber, the method comprising the steps of:
forming a plurality of ceramic greensheets wherein the ceramic material of the ceramic greensheets primarily comprises alumina particles, glass ceramic particles or aluminum nitride particles;
stacking and laminating the plurality of ceramic greensheets to form a stack with one of the plurality of ceramic greensheets being a first ceramic greensheet of the stack and one of the plurality of ceramic greensheets being a last ceramic greensheet of the stack;
applying a coating of ceramic material on at least one of the first and last ceramic greensheets of the stack, wherein the ceramic material of the coating is glass frit, grit, magnesium silicate, aluminum-magnesium silicate, aluminum-magnesium-silicon oxide, calcia-alumina-boria glass, or calcia-alumina-boria-yttria glass;
sintering the stack of ceramic greensheets at a predetermined temperature to cause densification of the ceramic material in the ceramic greensheets;
wherein the coating of ceramic material on at least one of the first and last ceramic greensheets of the stack alleviates camber resulting from sintering.
A fourth aspect of the present invention relates to a multilayer ceramic greensheet laminate comprising:
a plurality of ceramic greensheets stacked and laminated to form a ceramic greensheet laminate with one of the plurality of ceramic greensheets being a first ceramic greensheet of the stack and one of the plurality of ceramic greensheets being a last ceramic greensheet of the stack;
a coating of ceramic material on at least one of the first ceramic greensheet and the last ceramic greensheet.
A fifth aspect of the present invention relates to a multilayer ceramic substrate comprising;
a plurality of layers of ceramic material formed into a substrate, the substrate having a first surface and a second surface parallel to the first surface wherein the ceramic material of the substrate primarily comprises a first component and secondarily a second component; and
at least one of the first and second surfaces being richer in the second component than in the interior of the substrate so as to form a compositional gradient at the at least one of the first and second surfaces;
wherein the first component is alumina particles, glass ceramic particles or aluminum nitride particles and the second component is glass frit, grit, magnesium silicate, aluminum-magnesium silicate, aluminum-magnesium-silicon oxide, calcia-alumina-boria glass, or calcia-alumina-boria-yttria glass.